1. Field of the Invention
The present invention relates to a substrate processing method of and a substrate processing apparatus for processing substrates, such as semiconductor wafers, by, for example, a cleaning process.
2. Description of the Related Art
When processing a semiconductor wafer by, for example, a photolithographic process, a resist film is formed on a surface of the wafer, the surface of the wafer coated with the resist film is exposed to light in a desired pattern, and the wafer is subjected to a developing process to develop the image of the pattern, and then unnecessary portions of the resist film are removed.
A cleaning apparatus is employed for removing the unnecessary portions of the resist film. The cleaning apparatus immerses the wafer in a cleaning tank containing a chemical liquid called SPM (an H2SO4/H2O2 mixed liquid) to remove the portions of the resist film. In view of the protection of the environment, it is desired to remove the resist film by using ozonic water capable of being easily disposed of by a liquid disposal process instead of a chemical liquid. When ozonic water is used, the wafer is immersed in a cleaning tank containing ozonic water and the resist forming the resist film is decomposed into carbon dioxide, water, etc. by an oxidizing interaction between oxygen radicals contained in the ozonic water and the resist film.
Normally, ozonic water is prepared by dissolving a high-concentration ozone gas in pure water by bubbling. The ozone of the ozonic water contained in the cleaning tank escapes from the ozonic water and the ozone concentration of the ozonic water decreases gradually with time. Since the cleaning ability of the ozonic water is dependent on its ozone concentration, the cleaning ability of the ozonic water decreases as the ozone concentration decreases and the ozonic water having an insufficient ozone concentration is often unable to remove the resist film satisfactorily. Since the rate of interaction between ozone and the resist film is very high, only insufficient ozone can be supplied to the surface of the resist film if the wafer is merely immersed in the ozonic water and hence the interaction between the ozone and the resist film cannot be carried out at a high rate. The interaction between ozone and the resist film must be carried out at a high temperature. Therefore, in some cases, high-temperature ozonic water is prepared by dissolving ozone in pure water of, for example, 80° C. by bubbling. However, since the solubility of a gas in pure water at high temperature is low, ozonic water of high ozone concentration cannot be produced and hence the interaction between ozone and the resist film at a high reaction rate cannot be achieved.
Accordingly, it is an object of the present invention to provide a substrate processing method and a substrate processing apparatus capable of exercising a high processing ability in processing substrates.